The present invention is directed to the art of diagnostic imaging. It finds particular application in conjunction with CT scanners and will be described with particular reference thereto. However, it is to be appreciated that the invention may also find application in conjunction with magnetic resonance, positron emission, and other types of diagnostic imaging.
Heretofore, CT scanners have defined an examination region or scan circle in which the patient or other subject to be imaged was disposed. A beam of radiation was transmitted across the scan circle from an x-ray source to oppositely disposed radiation detectors. The segment of the beam impinging on a sampled detector defines a ray extending between the source and the sampled detector. The source or beam of radiation was rotated around the scan circle such that data from a multiplicity of rays crisscrossing the scan circle were collected.
The sampled data was convolved and backprojected into an image memory which was commonly described as a two dimensional array of memory elements. Each memory element stored a CT number indicative of the transmission or attenuation of the rays attributable to a corresponding incremental element within the scan circle. The data from each ray which crossed a given incremental element of the scan circle contributed to the corresponding CT number, i.e. the CT number for each memory element of the resultant image was the sum of contributions from the multiplicity of rays which passed through the corresponding incremental element of the scan circle.
Most commonly, the x-ray data was transformed into the image representation utilizing filtered backprojection. A family of rays was assembled into a view. Each view was filtered or convolved with a filter function and backprojected into the image memory. Various view geometries have been utilized in this process. As one example, each view was composed of the data corresponding to rays passing parallel to each other through the scan circle, such as from a traverse and rotate type scanner. In a rotating fan beam type scanner in which both the source and detectors rotate, each view could be made up of concurrent samplings of the detectors which span the x-ray beam when the x-ray source was in a given position, i.e. a source fan view. The detector commonly had either an equal linear spacing or an equal angular spacing. Alternately with a stationary detector rotating source geometry, a detector fan view could be formed from the rays received by a single detector as the x-ray source passed in back of the scan circle from that detector.
In forward projection, the image data was processed to synthesize each of the multiplicity of views that went through convolution and backprojection to make the image representation.
Various backprojection and forward projection algorithms have been developed. CT scanner customers normally demand a substantially instantaneous display of the resultant CT image. To obtain the image representation rapidly, the backprojection was normally performed with dedicated backprojection hardware. The many millions of computations required rendered general purpose computers inappropriately slow for backprojection. Various forward projection software routines have been written for general purpose minicomputers. However, the task of breaking the image representation apart into its component parts again requires many millions of computations. The execution time for such programs, commonly on the order of many minutes to hours, render forward projection too slow for many every day clinical applications.
In accordance with the present invention, a high speed backprojection/forward projection method and apparatus are provided.